Japanese Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Japanese language. It's 100% free, no registration required.

There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs.
If this question can be reworded to fit the rules in the help center, please edit the question.

4 Answers
4

Something to consider before getting into the particle combinations is the idea of は and が. There are a lot of different rules (sometimes seemingly-contradictory rules,) for は and が. But for the sake of simplicity, just remember that は is often the first, main subject of a sentence. が is kind of the second main subject (the "newer subject" or "focus point",) of a sentence.

In the first example, とは actually seems related to when って is used after dialog, like this: 「ボタン」って、何ですか。 ("Button" <- pointing to subject, what is?) It's a sort of marker for the previous word... something that makes a sort of subject out of the phrase before とは.

のは

When のは is used, it often refers to a subject (placed in front of のは,) for example:

勉強をするのは難しいことです。(i.e. Studying is hard. -or- The thing of studying is hard.) Using のは like this effectively turns a subject (a noun or a concept,) into a subject of the sentence.

のが

のが is similar to のは. But the difference comes down to the difference between は and が (explained earlier.)

So if we go back to the example used for のは and replace のは with のが:

私は勉強をするのが難しいことです。(i.e. For me, studying is hard. -or- For me, the thing of studying is hard.) Here, the main subject of the sentence is "私 myself". But when のが is used like this, のが (like のは）still effectively turns a subject (a noun or a concept,) into a subject of the sentence, but here, のが is pointing to a "new subject" of the sentence (the thing of studying.)

ことが

With the example given with のが, ことが is essentially the same thing. It creates a concept from a given noun, verb, or phrase.

The topic title is What is とは, ..., so I gave an answer to what とは is. How is that unrelated? Just because I didn't answer every question in the post doesn't mean my answer is unrelated. Same with @Ignacio's answer above. Check this post on meta. "So... in conclusion, don't hesitate to answer just because it might not be the perfect, complete, and final answer. Lots of answers are good. Even partial answers which address only one aspect of the issue are good."
–
istrasciFeb 9 '12 at 16:52

+1; this answer still relates to the question.
–
summeaFeb 27 '12 at 6:02

Hi! Sorry, I dont really understand what you are trying to say with "As for with X, Y" and "as for Xing, Y" but thanks anyway.
–
user1087Feb 7 '12 at 6:02

That's just the normal way to treat 「は」, but modified with the other particle.
–
Ignacio Vazquez-AbramsFeb 7 '12 at 6:03

2

Considering that all but one of the particle combinations in the question involves は, I think a good answer here has to go into a little detail on the contrasting function of は, or at least link to another answer that does.
–
HyperwormFeb 7 '12 at 13:09